An optical module, having a polarizing plate including a polarizer, and a light emitter(s) disposed in a position facing the polarizing plate, is incorporated and used in an optical device these days. Such an optical module is typically used in a display device, in particular a liquid crystal display device. The liquid crystal display device includes a liquid crystal display panel, and a surface light source device which functions as a backlight.
As shown in FIG. 40, the surface light source device includes a light source including light emitters, and a number of optical sheets for changing the traveling direction of light from the light emitters, and is designed to be capable of illuminating the liquid crystal display panel with desired optical characteristics. In the exemplary surface light source device shown in FIG. 40, a diffusion plate A, a lower diffusion sheet B, a light condensing sheet C and an upper diffusion sheet D are arranged in this order, starting from the member nearest to the light emitters 26 of the light source 25. The light condensing sheet C has a function (light condensing function) that narrows down the traveling direction of light to the front direction, thereby increasing the front direction luminance. On the other hand, the diffusion plate A, the lower diffusion sheet B and the upper diffusion sheet D have a light diffusing function that diffuses light from the light emitters 26 of the light source 25, thereby obscuring the images of the light emitters 26.
On the other hand, the liquid crystal display panel includes a liquid crystal cell 11 capable of controlling the orientation of a liquid crystal for each pixel, a lower polarizing plate 13 disposed on the light entrance side of the liquid crystal cell 11, and an upper polarizing plate 12 disposed on the light exit side of the liquid crystal cell 11, as shown in FIG. 40. The pair of polarizing plates 12, 13 includes a polarizer which transmits light of a particular polarization component and absorbs light of a component(s) other than the particular polarization component, and a protective film, bonded to the polarizer, for protecting the polarizer.
In general, the protective film is comprised of a mere light transmissive film due to cost constraints and does not positively exert an optical effect on transmitted light. There is a known protective film to which an optical function is imparted, but only to such an extent that the surface of the protective film, on the side not facing a polarizer, is matted, because of the constraints of the adhesion to the polarizer and the polarizer-protecting function (see e.g. JP 9-258013A).
However, a sufficient light diffusing function cannot be imparted to a protective film merely by matting the one surface of the protective film. Therefore, as shown in FIG. 40, it has been necessary for the conventional display device to have a number of optical sheets on the light entrance side of the polarizing plate.
The inclusion of a number of optical sheets in a surface light source device (display device) may cause various problems. Firstly, an increase in the number of optical sheets directly increases the production cost of the display device. Further, assembly of a surface light source device (display device) using a large number of optical sheets necessitates troublesome positioning between the optical sheets and between the optical sheets and light emitters, which may also increase the production cost of the display device.
Each optical sheet does not transmit all the incident light: part of the incident light is reflected by the optical sheet. Light reflected by an optical sheet may be reflected e.g. by a reflective plate 21 (see FIG. 40) provided behind the light emitters 26, or by another optical sheet and can thus be reused. However, every time light is reflected by an optical sheet, part of the light is absorbed by the optical sheet. Such a loss of light by reflection considerably increases even when the number of optical sheets merely increases by one. Thus, the use of a number of optical sheets in a surface light source device (display device) will significantly lower the efficiency of the use of light emitted by the light emitters of a light source.
An optical sheet can be heated by heat from light emitters, which may cause deformation of the optical sheet, such as bending, deflection or warpage. In such a case, adjacent optical sheets can make contact with or rub against each other in a surface light source device (display device) having a number of optical sheets. A contact area between optical sheets cannot exert the intended optical effect. Furthermore, there is a fear of the contact area being visible. In addition, rubbing between optical sheets can cause scratching in the sheet(s) or produce debris, leading to poor display image quality.